The Big Mystery of the Kuiper Belt — a Lack of Small Objects

The Kuiper Belt, a grouping of millions of rocky and icy bodies, encircles our Solar System like a giant doughnut larger than the orbit of Neptune. But, a new study shows that the belt contains far fewer small objects than would be expected in such a formation.

Pluto is the best-known of all bodies in the Kuiper Belt, and NASA’s New Horizons spacecraft was the first observatory to visit it, in 2015. Analysis of data from New Horizons shows that small craters are rare on Pluto and its largest moon, Charon. This suggests that small Kuiper Belt Objects (KBO’s), measuring between 300 feet and one mile in diameter, are few and far between in the system.

“These smaller Kuiper Belt objects are much too small to really see with any telescopes at such a great distance. New Horizons flying directly through the Kuiper Belt and collecting data there was key to learning about both large and small bodies of the Belt,” said Kelsi Singer, co-investigator of NASA’s New Horizons mission at the Southwest Research Institute (SwRI).

Charon, the largest moon of Pluto, is seen here by the New Horizons spacecraft in 2015. Note the smooth surface. Image credit: NASA-JHUAPL-SwRI

As the planets formed 4.6 billion years ago, worlds coalesced from dust and gas, in a process known as accretion. The Kuiper Belt is believed to be leftover remnants from this ancient process.

Examination of impact craters on bodies in the solar system provide valuable information about the formation and evolution of solar system. The asteroid belt between Mars and Jupiter contains a rich collection of small bodies, leaving researchers pondering why a similar distribution is not seen in the Kuiper Belt. One possibility is that the smaller, tighter formation in the asteroid belt results in more frequent collisions than seen in the Kuiper Belt, breaking larger objects up into smaller pieces.

Pluto is located so far from Earth that telescopes are unable to discern any detail on its frozen surface. The 2015 encounter between New Horizons and Pluto revealed a large heart-shaped feature, and mountains reaching four kilometers (13,000 feet) above the ground — nearly the height of Mauna Kea in Hawaii.

Mountains on Pluto viewed by New Horizons. Image credit: NASA-JHUAPL-SwRI

Unknown geological processes on Pluto have erased some features on that world, but Charon retains nearly all evidence of past impacts. The mountains of Pluto are believed to be no more than 100 million years old — one of the youngest planetary surfaces in the Solar System. Without a known source for internal heat, astronomers are left puzzling over what could be causing the reworking of the surface of this world.

“This breakthrough discovery by New Horizons has deep implications. Just as New Horizons revealed Pluto, its moons, and more recently, the KBO nicknamed Ultima Thule in exquisite detail, Dr. Singer’s team revealed key details about the population of KBOs at scales we cannot come close to directly seeing from Earth,” explained Alan Stern, also of SwRI, and principal investigator on New Horizons.

Discovered in 1930 by astronomer Clyde Tombaugh, Pluto was considered to be the ninth planet of the Solar System before it was reclassified as a dwarf planet in August 2006. In addition to Charon, Pluto is also accompanied by four other moons — Nix, Hydra, Styx, and Kerberos, each of which was discovered by astronomers using Earth-based telescopes.

New Horizons was launched in January 2006 on a mission to explore Pluto and other objects in the Kuiper Belt. When it passed Pluto in July 2015, it became the first (and so far only) spacecraft to explore that distant body.